The present invention relates to a crescent-less internal gear pump for producing high pressures, and having radial openings through the internally geared wheel into the tooth gaps between teeth of that wheel. A pump of this type is known from U.S. Pat. No. 2,915,982.
Such gear pumps have an internally toothed, internally geared wheel which surrounds an externally toothed pinion having a smaller number of teeth. The teeth in those gears are in driving engagement. As a general rule, the toothing of such pumps is relatively narrow in the axial direction of the pump, as compared to the diameter of the pinion or of the internally geared wheel. The volumetric flow to be conveyed by the pump is determined by the radial height of the teeth and by the width of the toothing in the axial direction of the pump. It is important that each cell which is formed by each tooth gap in the internally geared wheel and by the tooth of the pinion projecting into that tooth gap is filled as completely as possible with pressure fluid in the suction or intake region of the pump.
The known pump is provided with radial openings which extend from the outer circumference, or surface or periphery of the internally geared wheel into the tooth gap and through which pressure fluid can flow into the tooth gap. The internally geared wheel is, in this case, tightly surrounded by a sleeve which does not rotate with the internally geared wheel and which has a narrow slot for distributing the pressure fluid from the suction intake of the pump to the radial openings around the internally geared wheel. These radial openings, however, are of only slight axial length, along the direction of the axis of rotation of the pump, which is also referred to as the axial width of the internally geared wheel. Complete filling of the cells, therefore, appears possible only if the pump is operated at a particularly low speed of rotation. Otherwise, the pressure fluid must pass through the radial openings with a velocity of flow which is greater than the values which are favorable from a hydraulic fluidic standpoint.
If the velocity of flow of pressure fluid through the radial openings is in excess of an optimal value, which lies in the region of one meter per second, then the pressure in the flowing fluid assumes such low values that the air dissolved in the fluid emerges again. This leads to a strong formation of noise. Furthermore, this results in poor volumetric efficiency, leading the operator to select a larger pump when a given delivery volume is desired.
In the known pump, the internally geared wheel and the pinion are limited laterally by sealing plates which are pressed axially against the end sides of the pinion and of the internally geared wheel. Above a given pressure, these sealing plates move away axially from the gears so that the pump is not suitable for very high pressures, despite axial force application on the plates by springs or pistons.
It is true that it has been possible with the known pump to obtain a definite reduction in the outside dimensions, as compared with the internal gear pump known from U.S. Pat. No. 4,968,233. That known pump has cell space between the pinion and the internally geared wheel which is filled merely from both sides in the axial direction. The pressure fluid can flow in radially, laterally outside of the internally geared wheel. Therefore, the pinion and the internally geared wheel can have also only a small length in the axial direction. The toothed active rotating parts of the pump accordingly take up only a small portion of the structural length of the pump, while the remaining structural length is taken up by the housing with the lateral inflow of the pressure fluid. Therefore, even for a pump of low delivery volume, a large axial structural length must be tolerated. As compared with this, the pump in U.S. Pat. No. 2,915,982 is a considerable improvement. The construction of the internally geared wheel and of the suction port, however, still permits an expectation of conditions of flow which involve excessively high velocities of flow within the pump, and high pressures still cannot be obtained.